In semiconductor laser structures, feedback mechanisms for achieving a lasing condition have generally been in the form of reflective mirror facets. These facets are created by cleaving or polishing prescribed planar surfaces about the active laser cavity.
More recently, feedback has been obtained by a periodic variation of the refractive index within an optical waveguide abutting the laser cavity. Such periodic variations are produced by corrugating a surface at or near the core of the optical waveguide. Lasers which utilize these corrugated surfaces are called distributed feedback lasers or distributed Bragg reflector lasers. See, for example, U.S. Pat. No. 3,760,292 issued to H. Kogelnik et al. on Sept. 18, 1973 with respect to distributed feedback lasers and also see S. Wang, "Principles of Distributed Feedback and Distributed Bragg-Reflector Lasers", IEEE J. of Quantum Electronics, Vol. QE-10, No. 4, pp. 413-427 (1974).
Distributed feedback lasers incorporate periodic corrugations within, as well as outside of, an active gain medium. Distributed Bragg reflector lasers include the corrugations only in passive media adjacent to the active gain medium. As such, the periodic structures of the distributed Bragg reflector laser perform a function of frequency selective and reflection, in contrast to non-frequency selective end reflection performed by planar mirror facets. But, because each Bragg reflector is in a high loss passive waveguide medium outside of the pumped active medium, the net gain of the distributed Bragg reflector laser is less than the gain of the active medium.
Several refinements of the fundamental distributed Bragg reflector laser structure have been proposed of Wang in the aforementioned publication and by Y. Suematsu et al. in Japan. J. of Appl. Phys., Vol. 17, No. 9, pp. 1599-1603 (1978), and Elect. Lett., Vol. 16, No. 12, pp. 455-456 and pp. 456-458 (1980). In the Wang publication, the structure utilizes butt joint coupling of the active medium to the passive waveguide and reflector arrangements. Suematsu et al. employ an integrated twin guide structure which requires phase coupling to occur between the active medium and the passive waveguide and reflector arrangement thereunder. In both of these distributed Bragg reflector laser structures, the active and passive media are composed of semiconductor materials from the same family of compounds. Furthermore, in the passive waveguide media, the semiconductor material exhibits a transparent, rather than absorptive, property with respect to the frequency of the laser output. Although these refinements have tended to increase the differential quantum efficiency of these distributed Bragg reflector lasers over previous distributed Bragg reflector laser structures, they have not significantly reduced the loss of the passive (unpumped) waveguide media.